U.S. Pat. No. 4,903,308, which is incorporated herein by reference, discloses an audio transducer used o for producing mid-range to high range frequencies. This transducer has a pair of elongated resilient webs whose intermediate portions are joined together forming an expanse that extends generally in a plane, with the expanse supported for movement in the direction of the plane. This transducer is particularly well suited for high end consumer audio markets in which cost is not a substantial concern. Therefore, the complexity of the assembly and the precise manufacturing processes required do not prevent this transducer from being highly effective and marketable. In addition, overall efficiency of the existing transducer need not be maximized due to the generally adequate power capabilities of typical home audio amplifiers.
The foregoing transducer design is not as well suited for applications in which the manufacturing cost is critical and power is limited, as in portable stereo and car stereo applications. This is true of many other prior transducer designs as well. It is always desirable to reduce manufacturing cost and to increase efficiency for any application, particularly without sacrificing performance. Also, any transducer may be improved by widening its frequency range, especially by improving its high frequency efficiency.
A fundamental problem in extending the range of frequencies in any transducer is the seemingly unavoidable trade-off between the high and low frequency performance of the transducer. Measures to improve high frequency response, such as the use of lighter diaphragm materials, have the effect of diminishing output efficiency at the lower range of the transducer. Measures to improve low frequency response, such as the use of stiffer diaphragm materials, cause high frequency losses.
All prior art devices can benefit by reducing manufacturing costs. High performance transducers generally have numerous complex parts which must be carefully aligned in a labor- and skill-intensive manufacturing process that requires many assembly steps.
A further disadvantage of many prior transducers is that the speaker coil does not easily dissipate the heat that is generated when the transducer is driven under high load conditions. The coil is typically covered by material that thermally insulates the coil.
A further drawback in many prior transducers is the less than optimum high frequency efficiency due to the moving mass of the rigid portion of the diaphragm.
A further disadvantage in the prior art is the efficiency limitation caused by the lack of precision of alignment of the diaphragm relative to the magnet structure. To provide maximum efficiency, the magnets should be closely spaced adjacent the coil. This is especially critical with small, high frequency drivers, which typically use fewer coil turns and, thus, require a high strength magnetic field. The limitation of the prior art, however, is that imprecise positioning of the diaphragm and coil relative to the magnet creates a risk of the diaphragm contacting the magnet structure as the diaphragm vibrates or as misalignment occurs over time and use. Thus, a wide gap is required to tolerate imprecise alignment of the diaphragm and to prevent the unacceptably distorted output that occurs when the diaphragm contacts the magnet.
In the above the above-referenced prior art transducer, the diaphragms are aligned centrally in the magnet gap by a set of elastic cords, each spanning from one magnet to the other and passing through a small hole defined in the diaphragm. Although the elastic cords are sized to tightly fit the holes defined in the diaphragm, the diaphragm may slightly shift over time. This shift is tolerated by using a wider magnet gap, which results in a lower efficiency transducer unsuitable for applications such as automotive and portable stereos. An additional characteristic of this suspension approach is that the added mass of the elastic cords tends to slightly diminish the high frequency performance of the transducer.
A further characteristic of the prior art transducer making it less than ideal for portable applications, is the further reduced efficiency caused by the larger magnet pole plates, which must extend beyond the magnets to provide a rigid position for the magnets to be secured to each other across the magnet gap above and below the diaphragm, and without interfering with the diaphragm. The securing bars used for this purpose tend to limit the width of the diaphragm, resulting in limited efficiency.
A further disadvantage of all prior art audio transducers is that the diaphragm material has a less than desireable strength-to-weight ratio. In addition, the flexible materials such as the plastics and papers that are commonly used for such applications have a low resistance to solvents and acids and are highly susceptible to degradation in various types of radiation, particularly ultraviolet light as is found in outdoor applications, such as automotive installations.
A further disadvantage of the diaphragm materials used in the prior art is that the plastics and plastic coated papers commonly used have a surface that is generally incompatible with many adhesives, making manufacturing difficult by limiting adhesive choices to those adhesives with other undesirable properties.